Nouveau alpine chic: The green roof

Green roof A lush green roof can look like a shag carpet—but it does a whole lot more.

Reading up on the evolution of alpine architecture, I came across this: "Mountains were a place to die, not live or work or lounge about in an iridescent Bogner suit, sipping a martini. The great massifs of Europe were impassive killers that could take out an entire village with an avalanche without warning. So, when intrepid farmers began migrating higher up the range's flanks to graze their flocks and ultimately settle the first villages of the Alps, they built their homes as sanctuaries to withstand the treacherous wind, cold, snow and, yes, diabolical phantasms of the mountains."

One favoured way to guard against marauding spirits, it went on, was to build homes that blended into the surroundings. Roofing of local rock helped, as did roofs that looked like meadows, reflecting a trend I've recently noted in our own mountain town, albeit for more practical reasons—the green roof.

Green roofs aren't new. The Hanging Gardens of Babylon—constructed in the 7th century BC and one of the Seven Wonders of the Ancient World—are considered the first green roof. Built over stone arches waterproofed by layers of reeds and thick tar, they featured a variety of plants and trees. Later, indigenous cultures in Scandinavia covered rooftops in sod-over-bark to aid insulation, keeping homes cooler in summer and warmer in winter. Modern green roofs were developed in Germany in the early 1970s, spreading around Europe aided by technological advances. The past few decades have seen their popularity increase in North America as well—kickstarted by a trend toward rooftop gardens and urban ecosystems.

Comprising a system of layers to support soil medium and vegetation, green roofs fall into two general categories—intensive and extensive. Intensive roofs have deep soil layers, plus a wider variety of plants that add weight and require higher maintenance; the extensive type features fewer, thinner soil layers and smaller plants, hence less weight, less expense, less maintenance.

Whistler-based Custom Building Network, which specializes in challenging, non-status-quo projects, has developed expertise when it comes to green roofs.

"In 2008, we were working with a U.S. architect on a house project and they proposed a green roof," recalls principal Mike Ciebien. "So, I had to research the methods and products available. My concern was serviceability and warranty issues. In other words, would it stand up to Whistler's severe climate?"

At the time, Whistlerites might recall, both WAG and the Scandinave Spa had green roofs that were considered cutting edge—but they also weren't necessarily right for a house.

"Some green roofs can look like dried grasses for much of the summer and that's not a look we were after for this project," says Ciebien. "Then I discovered pre-grown roof systems. Some is sod you can unroll on your roof, but we ended up using a basket system from NATS Nursery in Vancouver in which plant sedums are specially selected and blended for their adaptation to an intended climate—such as being frozen solid in winter—then sprouted, grown and fully established before they're shipped."

Not only are these live baskets easy to fit onto a roof, but they transcend three issues that can limit self-establishing green roofs here: a short growing cycle; heavy rain events that can wash small plants and soils away before root systems are fully established; and the tendency of both these factors to allow grasses, weeds and invasive plants to gain foothold before intended plants do. Baskets also make for other conveniences.

"What I like most about the system is that if you need to service your roof you can just lift out the baskets, expose the membrane, find and rectify a problem, then put everything back together," notes Ciebien, who sees the biggest decision-maker for green roofs being looks. "As we see more contemporary homes built, more low-pitch or flat roofs are seen from upper floors of the house, another part of the property, or other parts of the neighbourhood. Green roofs are far more attractive than waterproofing membrane or pebbles."

Although there is some insulation value, it isn't significant given the high R-values builders must insulate to these days. Cooling the house in summer, however, is a definite plus. Other positive ecological benefits include: absorbing solar gain instead of radiating it into the surroundings (a bonus if a flat roof is being partially used as deck space); storm-water retention by soil and plants means run-off occurs over a longer period, putting less pressure on drain systems during heavy rains; the nursery mix strains are hardy and not only well-adapted to winter conditions, but bloom for much of the summer and are highly attractive to honeybees.

Green roofs are now required in some places in Europe and the technology is taking off. Holland is the world leader, using roofs to cultivate large amounts of food—far beyond that of the herb gardens of many B.C. restaurants. Given our growing season, turning Whistler's roofs into a future food source is doubtful, but who knows—having one might keep evil mountain spirits from noticing your house.

Leslie Anthony is a biologist, writer and author of several popular books on environmental science.